In a TDMA communication system, such as the European digital cellular radiotelephone systems which are set forth in the open air interface Global System for Mobile Communications (GSM) standard (Copies of GSM are available from European Telecommunications Standards Institute, ETSI Secretariat: B.P.152.F-06561 Valbonne Cedex, France) or a derivative thereof such as Digital Cellular System 1800 MegaHertz (DCS1800), there are typically a plurality of radio frequencies employed at each base site. Each frequency is time partitioned into frames, each having several time slots. Each time slot may carry radiotelephone communication between a particular base site and a subscriber unit (i.e., mobile communication unit) located within the coverage area of the particular base site.
In addition, the performance of TDMA communication systems can be improved by introducing a frequency hopping pattern to the signal modulation process. Frequency hopping helps a communication system maintain the integrity of a communication channel by providing frequency diversity. This, in combination with the channel coding and interleaving, mitigates the effects of Rayleigh fading. In addition, frequency hopping is an important counter-measure which reduces a channel's susceptibility to co-channel interference and jamming (which was intended or accidental in nature).
Frequency hopping consists of shifting the carrier frequency of a particular information signal in discrete increments in a pattern dictated by a code sequence or pattern. In particular, the transmitter jumps from frequency to frequency according to a code sequence. Frequency hopping communication systems can be divided into slow frequency hopping (SFH) and fast frequency hopping (FFH) communication systems, In SFH communication systems, several data symbols, representing a sequence of data bits (e.g., an information signal), modulate the carder wave within a single hop. Whereas, in FFH communication systems, the carrier wave hops several times per data symbol.
In an SFH communication system, multiple communication channels are accommodated by the assignment of portions of a broad frequency band and/or time slot to each particular channel. For example, communication between two communication units in a particular communication channel is accomplished by using a frequency synthesizer to generate a carrier wave in a particular portion of a predetermined broad frequency band for a brief period of time. The frequency synthesizer uses an input hopping code to determine the particular frequency from within the set of frequencies in the broad frequency band which the carrier wave will be transmitted on. Hopping codes are input to the frequency synthesizer by a hopping code generator. The hopping code generator is periodically clocked or stepped through different transitions which cause different or shifted hopping codes to be output to the frequency synthesizer. Therefore, as the hopping code generator is periodically clocked, then so too is the carrier wave frequency hopped or reassigned to different portions of the broad frequency band.
Multiple communication channels are allocated by using a plurality of hopping codes to assign portions of the frequency band to different channels during the same time period. As a result, transmitted signals are in the same broad frequency band of the communication channel, but within unique portions of the broad frequency band assigned by the unique hopping codes. These unique hopping codes preferably are orthogonal to one another for each cell or communication service region such that the cross-correlation between the hopping codes is zero.
In the GSM standard, a scheme for frequency hopping traffic channels in time slots is described. If this scheme is implemented significant performance improvements can be obtained. However, in the GSM system, the control channel is not designed to hop, because the system was optimized for a simplified acquisition by subscriber units to the communication system. Therefore, in order to provide simplified access, the control channel is transmitted at a fixed frequency so that it acts like a beacon for subscriber units to use in acquiring the communication system (e.g., the control channel beacon is used to frequency tune and time align the subscriber unit to the communication system). However, this makes the control channel vulnerable to jamming and Rayleigh fading. Therefore, a need exists for a scheme to accommodate frequency hopping of the control channel, while still allowing the mobile units to acquire the communication system.